Patent Grant
8084094
The present invention generally relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention relates to a coating system that inhibits the formation of deleterious phases in the surface of a superalloy that is prone to coating-induced metallurgical instability.
Certain turbine, combustor and augmentor components of gas turbine engines are susceptible to damage by oxidation and hot corrosion attack, and are therefore protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat. In combination, the TBC and bond coat form what has been termed a TBC system.
Environmental coatings and TBC bond coats in wide use include diffusion coatings that contain aluminum intermetallics (predominantly β-phase nickel aluminide (beta-phase NiAl) and platinum aluminides (PtAl)), and overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, rare earth metals, and/or reactive metals). Other types of environmental coatings and bond coats that have been proposed include beta-phase nickel aluminide (NiAl) overlay coatings. In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions (such as γ-Ni) containing intermetallic phases (such as beta-phase NiAl), beta-phase NiAl overlay coatings are predominantly the beta-phase NiAl intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Examples of beta-phase NiAl overlay coatings are disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, U.S. Pat. No. 6,291,084 to Darolia et al., and U.S. Pat. No. 6,620,524 to Pfaendtner et al. The suitability of environmental coatings and TBC bond coats formed of NiAlPt to contain the gamma-prime phase (γ′-Ni3Al) has also been considered, as disclosed in U.S. Patent Application Publication Nos. 2004/0229075 to Gleeson et al., 2006/0093801 to Darolia et al., and 2006/0093850 to Darolia et al. Aside from use as additives in MCrAlX overlay coatings, diffusion aluminide coatings, and gamma-prime phase NiAl coatings, platinum and other platinum group metals (PGM's) such as rhodium and palladium have been considered as bond coat materials. For example, commonly-assigned U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses PGM-based diffusion bond coats formed by depositing and diffusing platinum, rhodium, or palladium into a substrate surface, or alternatively diffusing a PGM into an otherwise conventional bond coat material.
TBC systems and environmental coatings are being used in an increasing number of turbine applications (e.g., combustors, augmentors, turbine blades, turbine vanes, etc.). The material systems used for most turbine airfoil applications comprise a nickel-base superalloy as the substrate material, a diffusion platinum aluminide (PtAl) as the bond coat, and a zirconia-based ceramic as the thermally-insulating TBC material. A notable example of a PtAl bond coat composition is disclosed in U.S. Pat. No. 6,066,405 to Schaeffer. Yttria-stabilized zirconia (YSZ), with a typical yttria content in the range of about 3 to about 20 weight percent, is widely used as the ceramic material for TBC's. Improved spallation resistance can be achieved by depositing the TBC by electron-beam physical vapor deposition (EB-PVD) to have a columnar grain structure.
Approaches proposed for further improving the spallation resistance of TBC's are complicated in part by the compositions of the underlying superalloy and interdiffusion that occurs between the superalloy and the bond coat. For example, the above-noted bond coat materials contain relatively high amounts of aluminum relative to the superalloys they protect, while superalloys contain various elements that are not present or are present in relatively small amounts in bond coats. During bond coat deposition, a “primary diffusion zone” of chemical mixing occurs to some degree between the coating and the superalloy substrate as a result of the concentration gradients of the constituents. At elevated temperatures, further interdiffusion occurs as a result of solid-state diffusion across the substrate/coating interface. The migration of elements across this interface alters the chemical composition and microstructure of both the bond coat and the substrate in the vicinity of the interface, causing what may be termed coating-induced metallurgical instability, often with deleterious results. For example, migration of aluminum out of the bond coat reduces its oxidation resistance, while the accumulation of aluminum in the substrate beneath the bond coat can result in the formation of topologically close-packed (TCP) phases that, if present at sufficiently high levels, can drastically reduce the load-carrying capability of the alloy. These detrimental effects occur whether the coating is used as a bond coat for a TBC, or alone as an environmental coating.
Certain high strength superalloys contain significant amounts of refractory elements, such as rhenium, tungsten, tantalum, hafnium, molybdenum, niobium, and zirconium. If present in sufficient amounts or combinations, these elements can reduce the intrinsic oxidation resistance of a superalloy and, following deposition of an aluminum-containing coating, promote the formation of a secondary reaction zone (SRZ) in which deleterious TCP phases form. An example of such a superalloy is commercially known as MX4, a fourth generation single-crystal superalloy disclosed in commonly-assigned U.S. Pat. No. 5,482,789 and exhibiting superior intrinsic strength relative to earlier-generation single-crystal superalloys. Other notable examples of high-refractory superalloys include single-crystal superalloys commercially known under the names René N6 (U.S. Pat. No. 5,455,120), CMSX-10, CMSX-12, and TMS-75, each of which has the potential for being prone to SRZ.
Significant efforts have been put forth to control SRZ in single-crystal superalloys. For example, commonly-assigned U.S. Pat. Nos. 5,334,263, 5,891,267, and 6,447,932 propose direct carburizing or nitriding of a superalloy substrate to form stable carbides or nitrides that tie up the high level of refractory metals present near the surface. Other proposed approaches involve blocking the diffusion path of aluminum into the superalloy substrate with a diffusion barrier coating, examples of which include ruthenium-based coatings disclosed in commonly-assigned U.S. Pat. No. 6,306,524 to Spitsberg et al., U.S. Pat. No. 6,720,088 to Zhao et al., U.S. Pat. No. 6,746,782 to Zhao et al., and U.S. Pat. No. 6,921,586 to Zhao et al. Still other attempts involve coating the surface of a high rhenium superalloy with chromides or cobalt prior to aluminizing the surface, as disclosed in U.S. Pat. No. 6,080,246. Finally, above-noted U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses that a PGM-based coating diffused directly into a superalloy substrate can eliminate the need for a traditional aluminum-containing bond coat and thereby avoid SRZ and TCP phase formation.
Notwithstanding the above, there are ongoing efforts to develop coating systems that substantially reduce or eliminate the formation of SRZ in high-refractory alloys.
The present invention provides a coating process and system for an article comprising a substrate formed of a metal alloy that is prone to the formation of SRZ as a result of containing at least one refractory metal.
The coating system includes an aluminum-containing overlay coating and a stabilizing layer between the overlay coating and the substrate. As such, the coating process generally involves forming the stabilizing layer on the surface of the substrate, and then depositing the aluminum-containing overlay coating on the stabilizing layer. The overlay coating contains aluminum in an amount greater by atomic percent than an amount of aluminum in the metal alloy of the substrate, such that there is a tendency for aluminum to diffuse from the overlay coating into the substrate. The stabilizing layer consists essentially of at least one platinum group metal (PGM), namely, platinum, rhodium, iridium, and/or palladium. The stabilizing layer is sufficient to control diffusion of aluminum from the overlay coating into the substrate and stabilize the substrate, so that the substrate remains essentially free of an SRZ that would be deleterious to the mechanical properties of the alloy.
A significant advantage of this invention is that the stabilizing layer reduces and can even eliminate the formation and growth of SRZ in high-refractory superalloys that are especially prone to SRZ formation. The stabilizing layer is also potentially effective against the formation of extensive TCP phases. Furthermore, the invention allows for the use of an aluminum-containing overlay coating capable for forming an alumina scale, such that the overlay coating is suitable for use as a bond coat for TBC adherence or as an environmental coating for surfaces not coated by a TBC. The stabilizing layer of this invention is believed to be capable of maintaining the aluminum reservoir within the overlay coating for oxidation resistance, and improving the performance of bond coat and environmental coating materials that contain relatively low levels of aluminum, including hypostoichiometric beta-phase nickel aluminide intermetallic materials.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
The present invention is generally applicable to components that operate within environments characterized by relatively high temperatures, and are therefore likely to be subjected to oxidation, hot corrosion, thermal cycling, and/or thermal stresses. Notable examples of such components include the high and low pressure turbine nozzles and blades, shrouds, combustor liners, and augmentor hardware of gas turbine engines. An example of a high pressure turbine blade 10 is shown in
Represented in
As is typical with TBC systems for components of gas turbine engines, the bond coat 24 is preferably an aluminum-rich composition. As used herein, an aluminum-rich composition generally denotes a coating that contains a greater amount of aluminum (in atomic percent) than the substrate it protects. Aluminum-rich coating compositions of particular interest to the invention contain about 16 to about 40 weight percent aluminum. Preferred compositions for the bond coat 24 are nickel aluminide intermetallic overlay coatings of predominantly the beta phase (β-NiAl intermetallic), such as greater than 50 volume percent and more typically greater than 80 volume percent beta phase, with the balance mainly the gamma prime phase (γ′-Ni3Al intermetallic) and possibly smaller amounts of alpha-Cr and Heusler (Ni2AlX) phases. In addition to nickel and aluminum, nickel aluminide intermetallics suitable for use as the overlay bond coat 24 may also contain additions of chromium, silicon, one or more reactive elements (e.g., yttrium, zirconium, hafnium, and cerium), one or more rare earth metals, and/or one or more refractory metals. Examples of suitable nickel aluminide intermetallic overlay coatings are disclosed in U.S. Pat. Nos. 6,153,313, 6,255,001, 6,291,084, and 6,620,524, which nominally contain, in atomic percent, about 30% to about 60% aluminum (about 16 to about 40 weight percent). Particularly suitable coatings contain about 30 to about 38 atomic percent aluminum (about 16 to about 22 weight percent), optionally up to about 10 atomic percent chromium, optionally about 0.1% to about 1.2% of a reactive element such as zirconium and/or hafnium, optional additions of silicon, and the balance essentially nickel. The bond coat 24 may have a thickness of about 12 to about 75 micrometers, though lesser and greater thicknesses are also possible. The bond coat 24 can be deposited by various overlay processes, such as physical vapor deposition (PVD) processes that include cathodic arc (ion plasma) physical vapor deposition, electron beam-physical vapor deposition (EBPVD), sputtering, and thermal spraying. It is worth noting here that overlay coatings are physically and compositionally distinguishable from diffusion coatings. A diffusion coating significantly interacts with the substrate it protects during deposition as a result of the diffusion process to form various intermetallic and metastable phases beneath the substrate surface, and therefore contains base metal constituents that may be undesirable from the standpoint of providing environmental protection to the substrate. In contrast, an overlay coating does not significantly interact with the substrate it protects during deposition, and as a result predominantly retains its as-deposited composition with a limited diffusion zone.
Aluminum-rich bond coats of the types described above naturally develop an aluminum oxide (alumina) scale 28, which can be more rapidly grown by selective oxidation of the bond coat 24. The ceramic layer 26 is chemically bonded to the bond coat 24 with the oxide scale 28. As shown, the ceramic layer 26 has a strain-tolerant structure with columnar grains produced by depositing the ceramic layer 26 using a physical vapor deposition technique known in the art (e.g., EBPVD), though a plasma spray technique could be used to deposit a noncolumnar ceramic layer. A preferred material for the ceramic layer 26 is an yttria-stabilized zirconia (YSZ), a preferred composition being about 6 to about 8 weight percent yttria, optionally with up to about 60 weight percent of an oxide of a lanthanide-series element to reduce thermal conductivity. Other ceramic materials could be used for the ceramic layer 26, such as yttria, nonstabilized zirconia, or zirconia stabilized by magnesia, ceria, scandia, and/or other oxides. The ceramic layer 26 is deposited to a thickness that is sufficient to provide the required thermal protection for the underlying substrate 22 and blade 10, generally on the order of about 75 to about 300 micrometers, though lesser and greater thicknesses are also possible. While described in reference to a coating system 20 that includes a ceramic layer (TBC) 26, the present invention is also applicable to coating systems that exclude a ceramic coating, in which case the bond coat 24 is the outermost layer of the coating system 20 and may be termed an environmental coating. However, for convenience the layer identified by reference number 24 in
As discussed previously, when deposited an overlay coating such as the bond coat 24 of
According to this invention, the bond coat 24 in
The predominant constituent of the stabilizing layer 42 is one or more platinum group metals (PGM's), more particularly platinum, rhodium, iridium, and/or palladium, and is therefore termed a PGM-based metallic material. More preferably, the stabilizing layer 42 is formed entirely of platinum, rhodium, iridium, and/or palladium, along with incidental impurities and elements inevitably present as a result of even limited interdiffusion with the bond coat 24 and the substrate 22. In atomic percent, the stabilizing layer 42 contains a combined amount of at least about 75% platinum group metal(s), and more preferably at least 90% platinum group metal(s), again along with incidental impurities and elements inevitably present as a result of even limited interdiffusion with the bond coat 24 and the substrate 22. Optionally, the stabilizing layer 42 could be alloyed to further contain intentional additions of nickel, cobalt, chromium, aluminum, and ruthenium in a combined amount of up to about 25 atomic percent. The stabilizing layer 42 can be formed by applying a layer of the platinum group metal or metals to the surface of the substrate 22, without performing a processing step to intentionally diffuse the layer into the substrate 22. For example, the platinum group metal or metals can be plated onto the surface of the substrate 22, followed by an optional heat treatment to remove hydrogen from the plated deposit and improve adhesion. The stabilizing layer 42 is preferably deposited before the bond coat 24 is deposited, and has a preferred final thickness of at least about three micrometers, more preferably about four to about twelve micrometers.
While not wishing to be held to any particular theory, the PGM stabilizing layer 42, as a result of being located between the SRZ-prone superalloy substrate 22 and the bond coat 24 with a higher aluminum content than the substrate 22, is believed to lower the activity of aluminum and be capable of promoting “uphill” diffusion of aluminum from the substrate 22 into the stabilizing layer 42. As such, the stabilizing layer 42 promotes the formation and subsequently helps to sustain a higher aluminum level region in contact with the substrate 22, while stabilizing the substrate against TCP formation. Furthermore, the aluminum contents in the substrate 22 and bond coat 24 remain relatively stable when the substrate 22 is subjected to high temperatures that would be otherwise sufficient to cause significant diffusion of aluminum from the bond coat 24 into the substrate 22 and lead to SRZ formation. Again, though not wishing to be held to any particular theory, the PGM stabilizing layer 42 is believed to reduce diffusion by reducing the activity of aluminum, in contrast to reducing diffusivity as is done with the use of a refractory element diffusion barrier layer.
In an investigation leading to the present invention, coatings in accordance with the foregoing discussion were deposited on SRZ-prone superalloy specimens and subsequently subjected to an extended high temperature exposure. The specimens were single-crystal castings formed of René N6 superalloy in the solutioned and primary aged condition. Some of the specimens were designated as experimental and provided with a stabilizing layer by plating an eight-micrometer thick layer of platinum on their surfaces, followed by a two-hour vacuum heat treatment at about 1700° F. (about 930° C.). The experimental specimens and the remaining baseline specimens were then coated with beta-phase NiAl intermetallic overlay coatings deposited by ion plasma deposition to a thickness of about thirty micrometers. The overlay coatings had the following nominal composition (in weight percent): about 18% aluminum, about 6% chromium, about 1% zirconium, and the balance nickel and incidental impurities. Finally, all specimens underwent a four-hour heat treatment at about 1975° F. (about 1080° C.).
The baseline and experimental specimens were then exposed at about 2050° F. (about 1120° C.) for about 50 hours to an air environment to assess the tendency for SRZ formation. Following this exposure, the specimens were sectioned and polished for metallographic viewing. The lefthand scanned image of
While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.
This is a division patent application of U.S. patent application Ser. No. 11/565,410, filed Nov. 30, 2006. The contents of this prior application are incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3999956 | Stueber et al. | Dec 1976 | A |
| 5334263 | Schaeffer | Aug 1994 | A |
| 7247393 | Hazel et al. | Jul 2007 | B2 |
| 20010002296 | Nazmy et al. | May 2001 | A1 |
| 20030022012 | Warnes et al. | Jan 2003 | A1 |
| 20030118863 | Darolia et al. | Jun 2003 | A1 |
| 20030152799 | Grossman et al. | Aug 2003 | A1 |
| 20050145503 | Kozlov et al. | Jul 2005 | A1 |
| 20080081214 | Narita et al. | Apr 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 0814178 | Dec 1997 | EP |
| 1327702 | Jul 2003 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20090220684 A1 | Sep 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11565410 | Nov 2006 | US |
| Child | 12465884 | US |
